Many medical diagnostic techniques project radiant energy into the body of an animal for testing for the existence of a biomedical disorder or condition. For example, the integrity of the skeletal structure may be examined by passing X-rays through the body. The dense bony material substantially blocks the passage of the X-rays, permitting a doctor or other medical care provider to visually inspect for fractures or other defects in the skeletal structure.
X-rays are also used in mammography to detect tumors in the breast. In this technique, the X-rays are passed through the breast such that they do not travel through any bony structure. The X-rays are hindered by tumors in the breast which show up as dark spots on the X-ray film. Although under this technique tumors may be detected earlier than by physical examination, the technique is difficult to administer due to the angle that the X-rays must be passed through the breast.
To examine the soft tissue of the body, other techniques are available. These include, among others, CAT scans and magnetic resonance imaging. Both project radiant energy onto the body for obtaining information about the physical structure of the body. Although these techniques are highly accurate and provide detailed information, they are time consuming to administer and costly to perform.
Further, measurement of the level of certain chemicals or compositions within the body is a diagnostic test of particular interest. Radiant energy may be used in these applications as passage of the radiant energy through particular chemicals or compositions often alters the radiant energy in a manner that can be measured and analyzed. For example, the monitoring of the glucose level of the blood is of particular importance to diabetics. One method of measuring the person's glucose involves projecting polarized radiation onto the body and measuring the optical rotation of the radiation that passes through the body. This optical rotation corresponds to the concentration of the glucose within the body. To be effective, however, the radiation must be passed through a relatively thin area of the body. Thus, the technique is usually performed on a person's earlobes or fingers. However, the skin and other tissue through which the radiant energy passes can interfere with the accuracy of the test.
To overcome the inaccuracies associated with passing the radiant energy through the tissue, attempts have been made to project the radiant energy through the cornea and aqueous humor of the eye. This is done because the concentration of glucose and oxygen in the cornea and aqueous humor, for example, reflects the concentration generally throughout the body. However, several problems are associated with this technique as well. For example, in Quandt U.S. Pat. No. 3,963,019, radiant energy is projected into the eye and reflected off the iris. The reflected radiation is detected and the optical rotation caused by passage of the reflected radiation through the cornea and aqueous humor is determined. However, this method suffers from poor sensitivity, in part because it relies on reflecting the radiant energy off the iris. Other attempts, as shown in March U.S. Pat. No. 3,958,560 and March U.S. Pat. No. 4,014,321, project the radiant energy at a shallow angle into the cornea on one side of the eye, through the aqueous humor, and out the cornea on the opposing side of the eye. Although this test is able to achieve high accuracy, it is difficult to administer because of the shallow angle at which the radiant energy must be passed through the eye.
Thus, there has been a considerable need for a relatively inexpensive device and method for reliably and easily performing tests using radiant energy for detecting the existence of biomedical disorders or conditions. More particularly, there has been a considerable need for a device and method for reliably and easily measuring the level of a substance in the body using radiant energy.